Method and apparatus for controlling gasifier efficiency

ABSTRACT

In a gasifier system, the gas thermal conductivity of the producer gas may beneficially be used as a control variable in controlling the combustion parameters of the gasifier process or gasifier system. For example, the control variable may be used to modulate the volume amount of air provided to a gasifier vessel or to modulate an oxidizer delivery rate.

RELATED APPLICATIONS

The present application claims priority to provisional Patent Application Ser. No. 61/072,471 filed Mar. 31, 2008, the disclosure of which is incorporated by reference herein, in its entirety.

SPONSORED RESEARCH AND DEVELOPMENT

This invention was supported in full or in part with an award from the Kentucky Science and Technology Corporation under the Kentucky Alternative Fuel and Renewable Energy Fund Program.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to gasifiers and, more particularly, with controlling operating efficiency of a gasifier.

2. Description of Related Art

Gasifier reactors are relatively simple in construction; however, the chemistry and physics involved in their operation can be quite complex and are still not well understood. Conventional thermal gasification processes oxidize a portion of a fuel feedstock (containing predominantly carbon, hydrogen, and in some cases oxygen) at an elevated temperature to provide heat energy in order to drive chemical reactions which convert carbon and hydrogen-containing compounds to carbon monoxide, hydrogen, carbon dioxide, and methane gas. Gasification takes place in two main steps. First, the fuel is partially combusted to form fuel gas and charcoal or char in a flaming pyrolysis zone. In the second step, a fraction of the carbon dioxide and water produced in the first step are chemically altered or reduced by the char, forming carbon monoxide and hydrogen. Gasification processes typically utilize temperatures above approximately 1400-1480° F. to minimize the formation of tars and high molecular weight hydrocarbons in the product gas. This gas, commonly called “producer gas” or “syngas”, contains hydrogen (18-30% by volume), carbon monoxide (18-25%), carbon dioxide (8-12%), methane (2-3%), water, nitrogen (if air is used as the oxidizing agent) and various contaminants such as small char particles, ash, tars and oils. The resulting producer gas is combustible with a low calorific value that can be used as a fuel for turbine and gas engines. Many gasification methods are available for producing fuel gas such as updraft, downdraft, and fluidized bed type processes.

The partial oxidation of the fuel feedstock can be carried out using air, oxygen, steam or a mixture there of. Air-blown gasification produces a low heating value gas (100-180 Btu/ft³ higher heating value) suitable for boiler, engine and turbine operation but not for pipeline transportation due to its low energy density. Oxygen gasification produces a medium heating value gas (250-500 Btu/ft³ higher heating value) suitable for limited pipeline distribution and as synthesis gas for conversion. Such a medium heating value gas can also be produced by pyrolytic or steam gasification.

The gasification of biomass, which includes crop residues, wood mulch, animal wastes, and manure, can be further complicated by the presence of sulfur, chlorine, and organic nitrogen. For example, poultry litter (PL), typically consists primarily of wood mulch, manure, and animal feed. Based on the ultimate analyses of PL, the simplified gasification reaction for can be summarized by the following stoichiometric equation:

C₁H_(1.6)O_(0.62)N_(0.1)S_(0.001)Cl_(0.001)+0.34O₂+1.27N₂ →aCO₂+(1−a)CO+bH₂O+cH₂ +dNH₃+0.001H₂S+(1.27+g)N₂

The yields and distribution of CO₂, CO, H₂O, and H₂ depend on numerous reactor operational and design parameters. Calculating the concentrations of these compounds in order to measure the caloric value of the producer gas as an indicator of the processes' efficiency can require complex and expensive techniques such as gas chromatography or mass spectroscopy which is unfeasible for many smaller-scale gasifier operations.

Thus, there remains a need for a simple, economical and speedy method for measuring and controlling the thermal efficiency of a gasifier system.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention relate to a method and apparatus that detects the thermal conductivity (TC) of a producer gas as a way to measure the thermal efficiency of a gasifier system and to control the efficiency of the gasifier system.

It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only various embodiments of the invention by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects of embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein:

FIGS. 1-4A are graphs depicting different characteristics related to gasifier efficiency.

FIG. 4B is a graph related to a relationship between producer gas thermal conductivity and gasifier temperature.

FIGS. 4C and 4D are, respectively, a schematic diagram and a graph of a control process in accordance with the principles of the present invention.

FIGS. 5 and 6 are diagrams of embodiments of the present invention implemented in a gasifier system.

DETAILED DESCRIPTION OF INVENTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the invention.

One useful parameter for characterizing a gasifier system is the quantity of air or other oxidizer supplied to the gasifier or the ratio of air rate to the fuel fed or consumed. Typically for air-blown gasification, the air-fuel ratio is expressed as a dimensionless number known as the air equivalence ratio, φ:

$\phi = \frac{m_{supplied}}{m_{stoich}}$

Where, m_(supplied) is the mass rate of the air supplied to the gasifier and m_(stoich) is the theoretical air mass rate required for complete combustion of the fuel. For gasification, φ is typically in the range of 0.25 to 0.5 depending on the fuel feedstock chemical composition and moisture content

Depending on the type of gasification plant, the overall gasification efficiency (i.e., the quality and quantity of the resulting fuel gas or producer gas) is a parameter that may beneficially be used by the gasifier control system. Gasification with air is the more widely used technology for small-scale applications since there is not the cost or hazard of oxygen production. Another quantity, the gasifier efficiency, describes the heating value of the gas stream in relation to that of the higher heating value of the fuel stream. With air gasification, the gasifier efficiency, is typically in the order of about 45 to 85%, typically about 70% for a relatively dry fuel.

The information beneficial for calculating the gasification efficiency includes: the energy content and volumetric rate of the resulting producer gas; the fuel consumption rate; and the fuel higher heating value. As mentioned earlier, quantifying the producer gas caloric value requires a relatively detailed chemical analysis of the gas. It is often assumed that the fuel consumption rate is equal to the fuel feed rate; however, this assumption is valid only for a gasifier operating at a steady-state condition. Additionally, the higher heating value of the fuel can vary substantially and requires off-line analytical tests.

The caloric value of the producer gas can be determined online by gas chromatography (GC) or by mass spectroscopy (MS) to quantify the concentrations of carbon monoxide, hydrogen, methane, and other hydrocarbons gases. Therefore, the considerable capital costs of this type of analytical equipment can make smaller-scale gasifier operations economically unfeasible. Therefore, embodiments of the present invention offer an alternative approach.

For example, embodiments of the present invention include a method for measuring the thermal efficiency of a gasifier system with a simple instrument that detects the thermal conductivity (TC) of the producer gas. One resulting benefit is that the lower heating value of the producer gas and fuel feedstock and the fuel consumption rate are not required for measuring gasification efficiency. The signal from a TC cell, relative to that of a reference gas such as air, is used to estimate the optimal operating conditions for maximizing gasification efficiency. Accordingly, the real-time signal from the TC cell can be used in conjunction with a process controller to modulate air or other oxidizer such that the efficiency of the gasifier may be optimized continuously. Additionally, the real-time signal from the TC could be used to shutdown the producer gas supply to heaters or generator engines if the quality of the gas is not to specification as a result of a process upset.

The thermal conductivities of the major gas compounds that make up a typical producer gas along with air are listed below in Table 1. Air is included in the table because it can be used as a reference gas. However, one of ordinary skill will recognize that other gases with known thermal conductivities may be used as reference gasses as well without departing from the scope of the present invention Embodiments of the present invention beneficially utilize the fact that the thermal conductivity of hydrogen is nearly an order of magnitude higher than that of any other of the producer gas constituents.

TABLE 1 Thermal Conductivity of the Major Producer Gas Compounds and Air. Gas Thermal Conductivity, Btu/hr-ft-° F. H2 CO CO2 CH4 N2 Air 0.0993 0.0145 0.0096 0.0195 0.0146 0.015

The total thermal conductivity of the gas produced will therefore vary in proportion to the hydrogen content. The art and science of thermal conductivity detectors (TCD) is known. For example, a TCD typically consists of a filament of platinum or tungsten wire situated in the effluent gas and heated by an electrical current. In the presence of the carrier gas alone the wire comes into thermal equilibrium at a given temperature. If placed in the arms of a Wheatstone bridge in which another arm contains a similar sensor situated in a reference stream of the same gas the bridge can be electrically balanced. When solute vapor is eluted from the column, the thermal conductivity and the specific heat of the gas surrounding the heated wire, changes. This change in thermal properties of the system results in a change in heat loss from the wire, a consequent change in wire temperature and wire resistance and the bridge becomes out of balance. The out-of-balance signal is amplified and fed to a recorder.

In the discussion that follows, a specific example of a biomass feedstock and other processing parameters are provided. This specific example is discussed so as to help illuminate embodiments of the present invention by way of a concrete example. However, embodiments of the present invention are not limited to only these specific values, parameters and materials discussed in the example.

The figures that follow relate to an example system that includes an air-blown adiabatic gasifier reactor using a wood biomass waste material containing 15 wt % moisture. The resulting producer gas can for example, be filtered and dried to a 120° F. dew point. One of ordinary skill, however, will recognize that other types of gasifiers, other biomasses, and producer gas handling techniques may be used without departing from the scope of the present invention.

FIG. 1 illustrates how the dry concentration or mole fraction of the main gas constituents such as hydrogen 102, carbon dioxide 104, and carbon monoxide 106, with a balance of methane 108, will vary according to the air or oxidizer equivalence ratio. The graph also reveals that, the thermal conductivity 110, also plotted in FIG. 1, is proportional to the hydrogen mole fraction.

Referring now to FIG. 2, the gasifier fractional efficiency 202 (with the same fuel basis as shown in FIG. 1) is plotted with the resulting lower heating value 204 of the producer gas versus the air equivalence ratio. While the heating value of the gas can be an important control parameter for engine applications, it is not a good indicator of the gasifier efficiency. In the example of the figures, the heating value decreases in proportion to the quantity of air added, thereby steadily increasing the concentration of nitrogen which dilutes the energetic compounds such as methane, carbon monoxide, and hydrogen.

In FIG. 3, the gasifier fractional efficiency 302 for the fuel is plotted with the gas thermal conductivity 304 versus the air equivalence ratio (or other oxidizer). The gasifier efficiency 304 is observed to track closely with the thermal conductivity 302. With a maximum efficiency and thermal conductivity occurring at an air equivalence ratio between about 0.35 and 0.40 in proportion to gas hydrogen mole fraction. Again, this graph relates to a specific example fuel and other graphs and values may be observed for different examples.

For example, referring now to FIG. 4 the same biomass previously considered has been further dried to an almost moisture-free state. Here the thermal conductivity 402 and the gasifier efficiency 404 peak more closely together at an air equivalence ratio of about 0.35. The conclusion supported by FIGS. 3 and 4 is that, in addition to the thermal conductivity signal indicating the hydrogen content of the fuel gas, it also provides useful information regarding the efficiency level of the gasifier. Furthermore, FIGS. 3 and 4 also show that relating gasifier efficiency and gas thermal conductivity is relatively insensitive to the moisture content of the fuel fed to the system. Thus, the gas thermal conductivity of the producer gas may beneficially be used as a control variable in controlling a gasifier process or gasifier system.

Much like the earlier figures, FIG. 4B is used to illustrate the principles of the present invention in one specific example. However, other curves and values would be utilized depending on the physical characteristics of the biomass, fuel, and oxidizer. Referring now to FIG. 4B, the control system beneficially maintains the exit gasifier temperature 408 within a bounded range between a low set point temperature (T_(sp low)) 412 in order to minimize tar and oil formation, and a maximum temperature set point (T_(sp high)) 410 to minimize high temperature materials corrosion and wear. Given a specific fuel composition and moisture content, the gasifier will exhibit equilibrium gasifier exit temperature 408 and gas thermal conductivity (TC) curves 414 for various air equivalence ratios (AER) as plotted in FIG. 4B. Here the dotted lines projected from the T_(sp high) 410 and T_(sp low) 412 temperature limits on the right-hand y-axis to the equilibrium temperature curve define the bounds of the allowable air input in terms of the AER. Since the composition and moisture content of biomass feedstocks can vary substantially over time, the relationship between the temperature and conductivity curves will also vary significantly with fuel changes. Therefore, the control system will continuously seek the highest attainable thermal conductivity while maintaining the gasifier exit temperature within the desired set point temperature limits.

In the example displayed in FIG. 4B, the allowable AER is in the range of 0.45 to 0.55 as bounded by the temperature limits. Although the thermal conductivity peaks at an AER of 0.38, the resulting equilibrium temperature would fall below the lower temperature set point. Therefore, the optimum TC 420 for control purposes (within the bounds of the temperature limits) occurs while operating the gasifier at the AER of 0.45. One challenge of implementing this type of control method is to adapt to changes in fuel composition. For example, if the fuel in the case of FIG. 4B were to become drier, the temperature curve would be shifted to the left, thereby necessitating a need to lower the AER for the given high and low temperature set points. Likewise, an increase in moisture would have the opposite effect. Consequently, the control system may constantly modulate the air or oxidizer flow rate to seek out the TC optimum. The control system should also be able to determine if the control action should be negative or positive. For example, if the gasifier is operating to the left of the TC signal maximum; the control action should be to increase the air flow rate, thereby increasing the air equivalence ratio. Oppositely, if the operating point is to the right of the TC maximum; the control action should be negative (decreasing air flow rate to lower the air equivalence ratio).

This type of control system can be shown schematically in FIG. 4C. This exemplary system is designed to be a cascade-type control with a variable gasifier temperature set point (for example, the temperature in the combustion zone or gas temperature of the gasifier). The value of this set point temperature may, for example, have a default value set to the low set low temperature limit (Tsp low) during the start-up phase of the gasifier operation. As the gasifier combustion zone begins to approach the initial set point temperature, the change in the conductivity with respect to the temperature gradient, λ, is calculated:

$\lambda = {\frac{\Delta \; {TC}}{\Delta \; T_{gas}} = \frac{{TC}_{\tau}\mspace{14mu} {TC}_{t - 1}}{T_{{gas}_{t}} - T_{{gas}_{t - 1}}}}$

Where TC, is the most recent thermal conductivity signal and TC_(t-1) is the signal from the previous sample period. Likewise, T_(gas t) and T_(gas t-1) are the most recent and previous temperature samples from the gasifier combustion zone, respectively. The sample timing interval may advantageously be between about 1 and about 10 seconds; although other time periods may be used without departing from the scope of the present invention.

The output signal 432 from the TCD device 430 measuring the producer or syngas gas 434 thermal conductivity is compared 436 to a set point constant that is predetermined to be the optimal for maximum hydrogen content and gasifier thermal efficiency. The control action (either direct or reverse acting) of the TC controller 438 depends on the sign of λ. If positive, the TC controller will proportionally increase the temperature set point 440 for the gasifier temperature controller. This control action would continue until the high temperature is obtained. If λ is negative, this would indicate that the controller is below the TC maximum; therefore, the set point signal is decreased proportionally until returning to the minimum temperature set point or until a TC maximum is found. For each case of λ, the amount of the temperature set point adjustment will depend on the error of the TCD signal with the optimum TC set point and the constants set in the proportional-integral-derivative (PID) TC controller.

The output of the TC controller 438 is a set point of temperature 440, within the bounds of the T_(sp low) and T_(sp high), to a gasifier temperature controller 442. This secondary controller 442 outputs a control signal to a final control element such as an air control valve 446 or air fan speed controller in order to regulate the mass of air entering the gasifier reactor and thus changing the AER. Hence, the gasifier process is influenced and a change in both producer gas temperature 448 and thermal conductivity (due to a change in hydrogen content in the gas) is measured. The control loop is completed by recalculating a new λ for the TC controller.

Referring now to FIG. 4D, the graph illustrates the operation of a gasifier system with a fuel the exhibits an equilibrium temperature 450 and a gas thermal conductivity 452 for varying AERs. As the gasifier operation is initiated, the control system may by default assigns an initial set point temperature of T_(sp Low). A predetermined TC set point is assigned for the TC primary controller that corresponds to the maximum hydrogen content that is attainable for the fuel. As the gasifier temperature reaches operating point 461, the control system calculates the change in TC with respect to the change in gas temperature, λ. In this case, since both the TC and temperature of the gas are increasing with respect to the AER, the sign of λ will be positive. Consequently, the primary TC controller will remain in a direct acting mode and will proportionally increase the set point temperature to the secondary temperature controller until operating point 462 is attained near the maximum of the TC curve. If operating point 462 is overshot, the sign of λ will be negative since the TC signal will be decreasing. Therefore, the control action of the TC controller will switch to a reverse acting mode and will proportionally decrease the temperature set point to the secondary gasifier temperature controller. This back-and-forth action around the TC maximum will continue if the TC set point is above that of the gasifier TC peak. Thus, the control system will continuously hover around the TC maximum in a metastable state. This metastable state is desirable since it will allow the control system to adapt to changes in the fuel such as variability of the moisture and/or hydrogen content such that it will seek a new maximum TC state. For example, if the fuel conditions change such that the system is near operating point 463 in FIG. 4D, the primary TC controller will calculate a negative λ, since the slopes of the TC and temperature curves are of opposite sign. Accordingly, the primary TC control would switch to the reverse-acting mode and would decrease the temperature set point to the secondary gasifier temperature controller until reaching a metastable state around operating point 2 near the maximum of the TC curve 452.

Referring now to FIG. 5, there is shown an embodiment integrated into a process control for a typical gasifier system 510. A small slip stream sample 512 is extracted at a fixed rate from the dried producer gas and may be heated above its dew point to avoid the formation of water droplets. The sample enters the thermal conductivity detector 514 where an output voltage signal is generated that is proportional to the gasifier efficiency. The control signal is then processed by a control system computer 516 along with the gasification temperature information 518. In practice, the process control software may be programmed to maintain a gasifier exit temperature between an acceptable low and high set point interval. The low temperature set point is set such that excessive tar and oil formation are avoided in the producer gas (e.g., a minimum of 1400° F. to 1480° F. is generally chosen depending on the fuel feedstock used). A high temperature set point is chosen to avoid excessive materials wear within the gasifier reactor. The control system provides a corresponding output to a motorized valve 520 or directly to a fan speed controller 522 to modulate the volumetric air or oxidizer rate such that the producer gas thermal conductivity (i.e., gasifier efficiency) is maximized within the desired temperature set point range within the gasifier reactor. For power applications, the gasifier control system could be programmed to switch a valve 524 such that the producer gas is sent to a flare or vent system in the event the desired caloric value specification (as detected by a low thermal conductivity) has not been met.

Referring now to FIG. 6, there is shown a schematic of the thermal conductivity detector system 650 with related equipment that achieves a proper gasification efficiency estimate. One of ordinary skill will recognize that various filters, valves and other components may be utilized and substituted without departing from the scope of the present invention. FIG. 6 merely shows many example components for one particular embodiment of the present invention.

The sampled producer 651 enters the TCD system via a pump 652 through a filter 654. The sample gas is modulated by a valve 656 and directed to a flow indication device 658 such as a rotameter. Simultaneously, a reference gas 671 such as air, is pumped through a similar network containing a pump 672, filter 672, metering valve 676, and reference flow measuring device 678. Beneficially, the flow rate of the sample and reference gas, should be equalized with their respective metering valves. The sample and reference gases may be heated within the analyzer enclosure and may be done so at a magnitude such that their temperatures are within one °F. as measured from temperature sensors 658 and 678. However, such heating is optional. The sample stream is piped via line 662 into a chamber 692 that contains an electrical resistive element such as a thermistor or a filament typically used in thermal conductivity detection devices. Likewise, the sample gas is piped via line 682 into a chamber 694 that contains a thermistor of substantially identical specifications to that of 692. For example, thermistors 682 and 692 may be designed to operate at temperatures between 120 and 212° F. The filaments or thermistors of the sample and reference cells can be connected in a Wheatstone bridge configuration 690 with substantially identical balance resistors 696 and 698. The thermistor or filaments are energized with a small electric direct current from a power supply 700 thereby heating the elements slightly above the temperature of the sample and reference gases.

The heat dissipated from the sample resistor 692 element will be proportional to the thermal conductivity of the fuel gas (compared to the air reference stream) with all other factors being substantially equal. Consequently, there will be a resistance difference between the reference and sample resistor elements in the respective conductivity cells. This resistance difference can be converted to a voltage signal via a voltage divider such as the Wheatstone bridge 690 which as understood by those skilled in electronics and thermal conductivity instrumentation. The resulting voltage imbalance is amplified via a difference operational amplifier circuit 710.

The signal output 712 has the advantage of providing relatively real-time (e.g., less than a 10 second response time) information regarding the producer gas quality and the gasifier efficiency. In comparison a state-of-the-art GC system that would have a sample response in the order of minutes and require expensive and difficult-to-operate equipment.

The previous description is provided to enable any person skilled in the art to practice the various embodiments described herein. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments. Thus, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the full scope consistent with each claim's language, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” 

1. A method for controlling a gasifier system, comprising: detecting a thermal conductivity of a producer gas from a gasifier vessel; and controlling an operating efficiency of the gasifier vessel based on the detected thermal conductivity.
 2. The method of claim 1, wherein controlling the operating efficiency includes controlling a volume of air provided to the gasifier vessel.
 3. The method of claim 1, wherein controlling the operating efficiency includes controlling a rate of oxidizer provided to the gasifier vessel.
 4. The method of claim 1, wherein the thermal conductivity is repeatedly detected so that controlling the operating efficiency is performed continuously.
 5. The method of claim 1, further comprising: drying the producer gas from the gasifier vessel before detecting the thermal conductivity.
 6. The method of claim 1, further comprising: determining a high temperature and a low temperature for combustion within the gasifier vessel, which bound an allowable air input to the gasifier vessel.
 7. The method of claim 6, wherein the high temperature and the low temperature relate to a fuel used in the gasifier vessel.
 8. The method of claim 6, wherein the high temperature and the low temperature relate to a moisture content of fuel used in the gasifier vessel.
 9. The method of claim 6, further comprising: determining an optimal thermal conductivity value within bounds of the high temperature and the low temperature; and controlling the operating efficiency so that the thermal conductivity of the producer gas is maintained substantially equal to the optimal thermal conductivity value.
 10. The method of claim 1, wherein the step of detecting thermal conductivity includes comparing the thermal conductivity of the producer gas with that of a reference gas.
 11. An apparatus for controlling a gasifier system, comprising: a gasifier vessel; a thermal conductivity detector configured to detect thermal conductivity of a producer gas from a gasifier vessel; and a controller configured to control an operating efficiency of the gasifier vessel based on the detected thermal conductivity.
 12. The apparatus of claim 11, wherein the controller includes a fan configured to control a volume of air provided to the gasifier vessel.
 13. The apparatus of claim 11, wherein the controller includes a valve configured to control a rate of oxidizer provided to the gasifier vessel.
 14. The apparatus of claim 11, further comprising: determination circuitry configured to determine a high temperature and a low temperature for combustion within the gasifier vessel, which bound an allowable air input to the gasifier vessel, the high temperature and the low temperature relating to a fuel used in the gasifier vessel.
 15. The apparatus of claim 14, wherein the controller is configured to control the operating efficiency so that the thermal conductivity of the producer gas is maintained substantially equal to an optimal thermal conductivity value, wherein the optimal thermal conductivity value is selected within bounds of the high temperature and the low temperature.
 16. A system for controlling a gasifier system, comprising: a producer gas path having a first input and a first output, the first input configured to receive producer gas from a gasifier vessel and the first output configured to provide a producer gas component to a thermal conductivity sensor; a reference gas path having a second input and a second output, the first input configured to receive a reference gas and the second output configured to provide an output gas component to the thermal conductivity sensor; the thermal conductivity sensor configured to provide an output signal related to a thermal conductivity of the producer gas component; and a control system configured to control operation of the gasifier system based on the output signal.
 17. The system of claim 16, wherein the thermal conductivity sensor includes a first thermistor whose value varies based on the thermal conductivity of the producer gas component and a second thermistor whose value varies based on a thermal conductivity of the reference gas component.
 18. The system of claim 16, wherein a respective temperature of the reference gas component and the producer gas component are substantially equal.
 19. The system of claim 16, further comprising: a vent configured to shunt the producer gas based on the thermal conductivity of the producer gas component being outside of an allowable range of values.
 20. The system of claim 16, wherein the control system includes a volumetric air controller configured to control air provided to a gasifier vessel. 